@require_POST
def cart_add(request, book_id):
    cart = Cart(request)
    book = get_object_or_404(Book, id=book_id)
    form = CartAddProductForm(request.POST)
    if form.is_valid():
        cd = form.cleaned_data
        cart.add(book=book, quantity=cd['quantity'], update_quantity=cd['update'])
    return redirect('cart:cart_detail')